1. Field of the Invention
This invention relates to the field of optical metrology in general, and to in-line thin-film reflectometry and profilometry for semiconductor wafers in particular.
2. Description of Related Art
A trend towards smaller critical dimension sizes in integrated circuits (IC) drives advances in technology for semiconductor capital equipment. Both technical factors, such as the ratio of the critical dimension size to the wavelengths of light used by fabrication device components, and well known economic factors, such as wafer throughput, Cost-Of-Ownership (COO) and Overall Equipment Effectiveness (OEE) are critical.
In IC fabrication, hundreds of process steps are necessary. During some of these steps, successive layers of materials are deposited on a substrate. Subsequently, Chemical Mechanical Polishing (CMP) is often used to make a film layer planar to high degree of precision. After a CMP process step, the thickness of the remaining film may be determined to verify that it is within desired tolerances.
Optical methods are commonly used to determine the thickness of thin films since light is generally non-destructive and non-invasive. Measured optical properties of the surface or measured wave-optics effects due to the interaction of light with thin films residing on the wafer yield desired information, such as film thickness. Thus, as critical dimensions on the wafer are reduced, there is a need for advances in optical metrology to obtain required precision and accuracy.
Technical requirements of precision and accuracy must be consonant with economic requirements. Fabrication machines must process wafers at a rapid rate with high uniformity and high reliability in addition to high precision. Since the fabrication must take place in a strictly controlled environment, the size of the machine is also an important factor. Easy operation is also important, despite the complexity of the processing and measurements. Performance in terms of these and other economic factors can be expressed through figures- of-merit such as COO and OEE.
Wafer metrology art comprises mostly “metrology mainframe” devices, which are devices only partially integrated with an IC fabrication line. There are at least two significant problems associated with partially integrated or non-integrated metrology control. First, waiting for test measurements from metrology mainframe systems to confirm the results from each process step is inherently inefficient. Second, with a partially integrated or non-integrated unit, process engineers face difficulties in achieving and maintaining optimal process parameters once they have the measurement information.
These and other problems associated with off-line metrology result in growing need for integrated (in-line) metrology in IC wafer fabrication. With in-line devices, the metrology apparatus is physically placed within the process equipment itself This enables a substantial reduction in times required to perform metrology measurements and shortens feedback or feedforward times between the metrology system and the process controls. By measuring critical parameters as each wafer is processed, the process equipment has information on the most recently processed wafer without stopping production. This results in good wafer-to-wafer control. Integrated metrology also significantly reduces operating costs by reducing the requirement for expensive test wafers, speeding up process qualifications and maintenance schedules, and provides an overall reduction in scrap wafers.
Related art in integrated thin-film metrology is limited regarding combining precise and accurate thin-film thickness measurements while meeting the other requirements of the semiconductor industry. Typically, related art in-line devices are limited to measurements of films of about 80 nm thickness. However, there is a need in the industry to measure film thickness of only a few tens of nanometers. Further, related art in-line devices are limited in their ability to make rapid, successive measurements over the totality of a wafer's surface.
What is needed is an imaging metrology system with rapid optical access to the entirety of a wafer surface. From the foregoing, it can be readily appreciated that many processes used in microelectronics manufacturing could benefit from integrated metrology, including but not limited to CMP, plasma etching, chemical vapor deposition, and lithography.